Electric power steering apparatus

ABSTRACT

An electric power steering apparatus including a torque sensor to detect a steering torque and a motor control unit to control a motor that applies an assist torque to a steering system of a vehicle, including: a function to switch a control system of the motor between a torque control system to control a motor output torque and a position/speed control system to control a steering angle of a steering in accordance with a predetermined switching trigger. The fade processing time from the torque control system to the position/speed control system and the fade processing time from the position/speed control system to the torque control system are individually set.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP205/083559, filed on Nov. 30, 2015, which claims priority from Japanese Patent Application Nos. 2014-244328 and 2014-244330, both filed on Dec. 2, 2014, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an electric power steering apparatus that has functions of an automatic steering control (an automatic operation mode, a parking assist mode, etc.) and a manual steering control and applies an assist force to a steering system of a vehicle by a motor, especially to an electric power steering apparatus having a function to switch a control system of the motor between a torque control system to control a motor output torque and a position/speed control system to control a steering angle of a steering.

The present invention further relates to an electric power steering apparatus where a fade processing time (a gradual changing time) from a torque control (a normal control) to a position/speed control of an automatic steering and a fade processing time (a gradual changing time) from a position/speed control to a torque control are individually set.

BACKGROUND ART

In an electric power steering apparatus (EPS) which has a motor control unit and applies a steering assist force to a steering system of a vehicle by a rotational force of a motor, the steering assist force is applied to a steering shaft or a rack shaft by a transmission mechanism such as gears and a belt with driving power of the motor via a reducer. Such the conventional electric power steering apparatus performs a feedback control of a motor current in order to precisely generate a torque of steering assist force. The feedback control is to adjust an applied voltage to the motor such that a difference between a steering assist command value (current commanded value) and a detected value of the motor current becomes small. Adjustment of the applied voltage to the motor is generally performed by adjusting a duty in a pulse width modulation (PWM) control.

Explaining a general configuration (column system) of an electric power steering apparatus with illustration in FIG. 1, a column shaft (steering shaft) 2 of a handle (steering wheel) 1 is connected to steered wheels 8L and 8R via reduction gears 3, universal joints 4 a and 4 b, a pinion and rack mechanism 5, tie rods 6 a and 6 b, and hub units 7 a and 7 b. A column shaft 2 is provided with a torque sensor 10 that detects a steering torque Ts of the handle 1. A motor 20 that assists the steering force of the handle (steering wheel) 1 is connected to the column shaft 2 via the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a steering assist command value of an assist (steering assist) command based on the steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by the vehicle speed sensor 12 and controls a current to be supplied to the motor 20 by a voltage control value Vref, which is the steering assist command value after compensation or other processing. Note that a steering angle sensor 14 is not a requirement and may not be disposed. The steering angle may be acquired by a rotational sensor connected to the motor 20.

The control unit 30 is connected with a controller area network (CAN) 40 that receives various information of the vehicle and the vehicle speed Vs can be received from the CAN 40. The control unit 30 may also be connected with a Non-CAN 41 that receives communication, an analog/digital signal, radio waves, or others that are different from those received by the CAN 40.

In such an electric power steering apparatus, the control unit 30 mainly includes a CPU (including an MPU, an MCU, etc.). Functions executed by a program inside the CPU are illustrated as an exemplary configuration as illustrated in FIG. 2.

Functions and operations of the control unit 30 are described with reference to FIG. 2. The steering torque Ts from the torque sensor 10 and the vehicle speed Vs from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Ts and the vehicle speed Vs using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.

The subtraction result I (=Irefm−Im) at the subtracting section 32B is proportional and integral (PI)-controlled at a PI-control section 35. The PI-controlled voltage control value Vref is inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.

The compensating section 34 adds a self aligning torque (SAT) 34-3 detected or estimated and an inertia compensation value 34-2 at an adding section 34-4. The addition result is further added with a convergence control value 34-1 at an adding section 34-5. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the characteristics.

In such an electric power steering apparatus, vehicles that have an automatic steering assist function (an automatic operation, a parking assist, etc.) and switches between the automatic steering control and the manual steering control have emerged in recent years. The vehicles having the automatic steering assist function perform the automatic steering control to set a target steering angle based on data from a camera (image), a distance sensor, or other apparatus and to cause an actual steering angle to follow the target steering angle.

In the automatic operation, environment surrounding the vehicle is recognized based on information from a radar, a camera, an ultrasonic sensor or the like and a steering angle command value that allows for safely guiding the vehicle is outputted. The electric power steering apparatus is capable of the automatic operation by performing a position control of the actual steering angle in such a manner as to follow the steering angle command value.

In the known electric power steering apparatus having the functions of the automatic steering control and the manual steering control in the related art, for example a back-in parking or a parallel parking is automatically performed by controlling an actuator (motor) based on relationship between a pre-stored traveling distance of the vehicle and a turning steering angle. That is, an automatic steering control apparatus recognizes a parking space from a positioning sensor such as an around-view monitor or an ultrasonic sensor and outputs a steering angle command value to the EPS-side. The EPS performs a position-control on the actual steering angle in such a manner as to follow the steering angle command value. As a result of this, the vehicle is guided into the parking space.

FIG. 3 is a diagram illustrating a control system of an electric power steering apparatus having the automatic steering control function. An automatic steering command unit 50 is inputted with various data from a camera and a positioning sensor (ultrasonic sensor or the like). A steering angle command value θtc for automatic steering is inputted into a position/speed control section 51 in an EPS-actuator function via a CAN or the like, and an automatic steering execution command is inputted into an automatic steering execution judging section 52 in the EPS-actuator function via the CAN or the like. The steering torque Ts is further inputted into the automatic steering execution judging section 52. An actual steering angle θr from the EPS-sensor is inputted into the position/speed control section 51 and a judgment result from the automatic steering execution judging section 52 is inputted into a torque command value gradual-change switching section 54. Further, the steering torque Ts from the EPS-sensor is inputted into a torque control section 53 in an EPS-power assist function, and a steering assist torque command value Tc from the torque control section 53 is inputted into the torque command value gradual-change switching section 54. A position/speed control torque command value Tp from the position/speed control section 51 is also inputted into the torque command value gradual-change switching section 54. According to the judgment result (ON/OFF of the automatic steering command) from the automatic steering execution judging section 52, the steering assist torque command value Tc and the position/speed control torque command value Tp are switched and output as a motor torque command value, thereby performing the drive-control of the motor via a current control system.

In this manner, a normal power assist is subjected to a torque control system. Meanwhile, the automatic operation such as the parking assist is subjected to a position/speed control system of the steering angle or other parameters. There are problems such as that the control torque varies upon the switching between the torque control and the position/speed control, thereby making the switching over not smooth and that an unintentional self-steer occurs by a trigger due to variations in the torque upon the switching over.

To handle such problems, a conventional method to gradually change (gradual-change) the control torque in the torque control and the position/speed control is used in order to mitigate the torque variations. For example in Japanese Unexamined Patent Publication No. 2004-17881 A (Patent Document 1), when an automatic steering mode is released at a time point t0 as illustrated in FIG. 4, “Sθ=OFF” is reset and thereafter an angle control ratio μ is monotonously reduced within a predetermined time ΔT. This allows a command value of a current to be conducted in a motor not to drastically vary even upon switching between the control systems.

THE LIST OF PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Publication No.     2004-17881 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Upon switching between the torque control and the position/speed control, however, the above effect cannot be fully exercised. This reason is because there are cases where assisting in an opposite direction occurs upon switching over to the normal power assist control since the position/speed control assists torque in such a manner as to suppress an external disturbance in a system such as the electric power steering that allows for inputting the external disturbance from the handle.

In this manner, the method to gradually change (gradual-change) the control torque in the torque control and the position/speed control is conventionally used in order to mitigate the torque variations. However, this cannot fully exercise its effect in the case of switching between the torque control and the position/speed control.

The present invention has been devised in consideration to the above circumstances with an object to provide an electric power steering apparatus capable of smoothly switching the control systems without self-steer by gradually changing a control torque of the torque control and a command value of the position/speed control upon fade processing (gradual-change processing) that switches the control systems.

Means for Solving the Problems

The present invention relates to an electric power steering apparatus including a torque sensor to detect a steering torque and a motor control unit to control a motor that applies an assist torque to a steering system of a vehicle, the above-described object of the present invention is achieved by that comprising: a function to switch a control system of the motor between a torque control system to control a motor output torque and a position/speed control system to control a steering angle of a steering in accordance with a predetermined switching trigger.

Further, the above-described object of the present invention is more effectively achieved by that wherein the predetermined switching trigger is ON/OFF of an automatic steering command; or wherein the predetermined switching trigger is ON/OFF of a switching command given by an internal judgment of the steering torque; or wherein, when the automatic steering command is turned ON, a fade processing is started and a post-gradual change steering-angle command value in a position/speed control is gradually changed from an actual steering angle to a steering angle command value, and a level of the assist torque in a torque control is gradually changed from 100% to 0% and then the position/speed control system is operated; or wherein, when the automatic steering command is turned OFF, a fade processing is started and a post-gradual change steering-angle command value in a position/speed control is gradually changed from a steering angle command value to an actual steering angle, and a level of the assist torque in a torque control is gradually changed from 0% to 100% and then the torque control system is operated, or wherein the post-gradual change steering-angle command value in the position/speed control is gradually changed by an exponential curve, and the level of the assist torque is gradually changed linearly; or wherein a fade characteristic of the fade processing can be freely tuned; or wherein a fade processing time 1 from the torque control system to the position/speed control system and a fade processing time 2 from the position/speed control system to the torque control system are different; or wherein the fade processing time 2 is shorter than the fade processing time 1; or wherein the predetermined switching trigger is performed by an automatic steering execution judging section; or wherein the automatic steering execution judging section comprises: a calculating section to calculate an angular speed and an angular acceleration by inputting a steering angle command value; a map judging section to judge each of the steering angle command value, the angular speed and the angular acceleration with a judging map corresponding to a vehicle speed; and a diagnosing section to diagnose based on a judgement result from the map judging section; or further comprising an external disturbance observer to compensate inertia and friction of a handle; or wherein the external disturbance observer estimates an external-disturbance estimation torque from a difference between an output of a steering inverse model of the steering system and an output of an LPF to limit a band; or wherein values of inertia and friction of the steering system are greater than or equal to values of inertia and friction of the steering inverse model, respectively.

Effects of the Invention

According to the electric power steering apparatus of the present invention, a post-gradual change steering angle command value is gradually changed from the actual steering angle to the steering angle command value and the actual steering angle is subjected to the position/speed control in such a manner as to follow the post-gradual change steering angle command value. This allows the torque command value in the position/speed control to be changed automatically and smoothly, thereby providing a soft handling feeling to a driver.

Further, even when excessive variations in the steering torque occur upon a fade processing of switching from the automatic steering to the torque control, the steering torque variations are automatically compensated by the position/speed control since the post-gradual change steering angle command value is gradually changed from the steering angle command value to the actual steering angle. Therefore, it is possible to suppress such a failure as to losing the control of the handle by the driver.

Moreover, as compared to a fade processing time to perform the fade processing from the torque control of the normal steering to the position control of the automatic steering, a fade processing time from the position control to the torque control is shorter. Therefore, there are advantages that the control is switched over relatively slowly so that the driver does not feel uncomfortable upon the fade processing from the torque control to the position/speed control and that the control can be switched over in a short time and intention of the driver can be promptly conveyed for avoiding danger upon the fade processing from the position/speed control to the torque control.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a configuration diagram illustrating an overview of an electric power steering apparatus (column system);

FIG. 2 is a block diagram illustrating an exemplary configuration of a control system of the electric power steering apparatus;

FIG. 3 is a block diagram illustrating an exemplary configuration of a control system of the electric power steering apparatus having a parking assist mode (automatic steering) function;

FIG. 4 is a characteristic diagram illustrating operation system of a conventional electric power steering apparatus;

FIG. 5 is a configuration diagram illustrating an overview of an electric power steering apparatus (single pinion system);

FIG. 6 is a configuration diagram illustrating an overview of an electric power steering apparatus (dual pinion system);

FIG. 7 is a configuration diagram illustrating an overview of an electric power steering apparatus (dual pinion system (exemplary variation));

FIG. 8 is a configuration diagram illustrating an overview of an electric power steering apparatus (coaxial rack system);

FIG. 9 is a configuration diagram illustrating an overview of an electric power steering apparatus (rack offset system);

FIG. 10 is a block diagram illustrating an exemplary configuration of the present invention;

FIG. 11 is a block diagram illustrating an exemplary configuration of an automatic steering execution judging section;

FIGS. 12A to 12C are characteristic diagrams illustrating exemplary judging maps (steering angle command value, angular speed and angular acceleration);

FIG. 13 is a diagram illustrating relationship between an example of mounting sensors and an actual steering angle used in the present invention;

FIG. 14 is a flowchart illustrating exemplary operations of the present invention;

FIG. 15 is a flowchart illustrating a part of exemplary operations of the automatic steering judging section;

FIG. 16 is a timing chart illustrating exemplary operations of the present invention;

FIGS. 17A and 17B are characteristic diagrams for explaining effects (fade processing) of the present invention;

FIGS. 18A and 18B are characteristic diagrams for explaining effects (fade processing) of the present invention;

FIG. 19 is a timing chart illustrating exemplary operations of another embodiment of the present invention;

FIG. 20 is a block diagram illustrating an exemplary configuration of an external disturbance observer; and

FIGS. 21A and 21B are characteristic diagrams illustrating exemplary effects of providing the external disturbance observer.

MODE FOR CARRYING OUT THE INVENTION

In a conventional torque gradual-change control in the electric power steering apparatus, there are problems such as that control is not smoothly switched upon switching between a torque control and a position/speed control and that unintentional self-steer occurs. In the present invention, therefore, a processing that smoothly switches the control without self-steer is implemented by gradually changing a control torque of a torque control and a command value of a position/speed control.

The present invention includes a function to switch control systems of a motor between a torque control system to control a motor output torque and a position/speed control system to control a steering angle upon steering in accordance with a predetermined switching trigger (e.g. an automatic steering command) and implements smooth a fade processing without self-steer.

Further, in the present invention as compared to a fade processing time (e.g. 500 to 1000 [ms]) to perform a fade processing from the torque control of the normal steering to the position control of the automatic steering, a fade processing time (e.g. 20 to 100 [ms]) from the position control to the torque control is set shorter. Accordingly, the control is switched over relatively slowly so that a driver does not feel uncomfortable upon the fade processing from the torque control to the position/speed control and the control can be switched over in a short time and intention of the driver can be promptly conveyed for avoiding danger upon the fade processing from the position/speed control to the torque control.

Furthermore, the present invention provides an external disturbance observer to compensate inertia or friction of the handle and therefore this allows a driver to easily intervene in the automatic steering by steering.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

The present invention may be applied to, other than the column system shown in FIG. 1, a single pinion system a schematic configuration of which is shown in FIG. 5, a dual pinion system an overview of which is shown in FIG. 6, a dual pinion system (exemplary variation) a schematic configuration of which is shown in FIG. 7, a rack coaxial system an overview of which is shown in FIG. 8, and a rack offset system an overview of which is shown in FIG. 9. The below descriptions will be given on the column system.

FIG. 10 is a diagram illustrating an exemplary configuration of the present invention, and the steering torque Ts is inputted into a torque control section 102 and an automatic steering execution judging section 120. A steering assist torque command value Tc from the torque control section 102 is inputted into a torque gradual-changing section 103. A steering angle command value θtc from a CAN or the like is inputted into the automatic steering execution judging section 120, the steering angle command value θt after calculation processing at the automatic steering execution judging section 120 is inputted into a steering-angle command value gradual-changing section 100, and a post-gradual change steering-angle command value θm from the steering-angle command value gradual-changing section 100 is inputted into a position/speed control section 101 together with an actual steering angle θr. A steering-assist torque command value Tg after the torque gradual-change and a position/speed control torque command value Tp from the position/speed control section 101 are inputted into an adding section 104 and the addition result from the adding section 104 is outputted as a motor torque command value. The motor torque command value is inputted into a current control system 130, and a motor 131 is driven and controlled through the current control system 130.

The automatic steering execution judging section 120 outputs ON/OFF of the automatic steering command being a judgment (diagnosis) result. The “ON/OFF” of the automatic steering command is inputted into the torque gradual-changing section 103 and the steering-angle command value gradual-changing section 100.

The automatic steering execution judging section 120 has a configuration as illustrated in FIG. 11, the steering angle command value θtc is inputted into a calculating section 121, and the calculating section 121 calculates an angular speed ωtc and an angular acceleration αtc based on the steering angle command value θtc. The angular speed ωtc and the angular acceleration αtc are inputted into a map judging section 122 to judge using a judging map. The map judging section 122 is also inputted with the steering angle command value θtc and a vehicle speed Vs. The map judging section 122 includes a judging map #1 for a steering angle command value θtc having a characteristic A1 or B1 as shown in FIG. 12A, a judging map #2 for an angular speed ωtc having a characteristic A2 or B2 as shown in FIG. 12B, and a judging map #3 for an angular acceleration αtc having a characteristic A3 or B3 as shown in FIG. 12C.

The characteristic of the judging map #1 with respect to the steering angle command value θtc is at a constant value θtc₀ until a vehicle speed Vs₁ of a low speed and decreases as the characteristic A1 or B1 in a range more than or equal to the vehicle speed Vs₁. The characteristic of the judging map #2 with respect to the angular speed ωtc is at a constant value ωc₀ until a vehicle speed Vs₂ of a low speed and decreases as the characteristic A2 or B2 in a range more than or equal to the vehicle speed Vs₂. Further, the characteristic of the judging map #3 with respect to the angular acceleration αtc is at a constant value αc₀ until a vehicle speed Vs₃ of a low speed and decreases as the characteristic A3 or B3 in a range more than or equal to the vehicle speed Vs₃. Any of the characteristics of the judging maps #1 to #3 can be tuned, and the characteristic may linearly decrease.

The map judging section 122 judges whether the steering angle command value θtc exceeds the range of characteristic values of the judging map #1, whether the angular speed ωtc exceeds the range of characteristic values of the judging map #2, and further whether the angular acceleration αtc exceeds the range of characteristic values of the judging map #3. A judgment result MD is inputted into a diagnosing section 123. The diagnosing section 123 outputs “ON/OFF” of the automatic steering command based on a diagnosis result by time or times (number) and “ON/OFF” of the automatic steering command is also inputted into an output section 124. The output section 124 outputs the steering angle command value θt only when the automatic steering command is “ON”.

Although the steering angle command value θt is inputted into the steering-angle command value gradual-changing section 100 together with the actual steering angle θr, the actual steering angle θr is calculated in the following manner in the present invention.

In a mechanism including a torsion bar 23, for example a sensor as illustrated in FIG. 13 is mounted to a column shaft 2 (2A (input side) and 2B (output side)) and thereby the steering angle is detected. That is, the input shaft 2A on a handle 1 side of the column shaft 2 is mounted with a Hall IC sensor 21 as an angle sensor and a 20° rotor sensor 22 for a torque sensor input-side rotor. The Hall IC sensor 21 outputs an AS_IS angle θh with 296° period. The 20° rotor sensor 22 mounted on the handle 1 side with respective to the torsion bar 23 outputs a column input-side angle θs with 20° period and the column input-side angle θs is inputted into a steering angle calculating section 132. The output shaft 2B on the column shaft 2 is mounted with a 40° rotor sensor 24 for a torque sensor output-side rotor. The 40° rotor sensor 24 outputs a column output-side angle Go and the column output-side angle Go is inputted into the steering angle calculating section 132. The column input-side angle θs and the column output-side angle signal Go are both calculated into an absolute angle by the steering angle calculating section 132. The steering angle calculating section 132 then outputs a steering angle θr on the column input-side and a steering angle θr1 on the column output-side of an absolute value.

Although the present invention descriptions are given assuming that the steering angle θr on the column input-side is the actual steering angle, the steering angle θr1 on the column output-side may be used as the actual steering angle.

Exemplary operations in such a configuration will be described with reference to flowcharts in FIG. 14 and FIG. 15 and a timing chart in FIG. 16.

When the automatic steering command is not “ON” (Step S1), the normal steering with the assist torque level of 100%, that is, the torque control is performed (Step S15). Then, when the automatic steering execution judging section 120 turns “ON” the automatic steering command at a time point t2 (Step S1), a fade processing of the EPS is started from the time point t2 (Step S2). At this time, “ON/OFF” of the automatic steering command is outputted from the automatic steering execution judging section 120, and the steering angle command value gradual-changing section 100 gradually changes the post-gradual change steering-angle command value θm of the position/speed control from the actual steering angle θr to the steering angle command value θt (Step S3). In the torque control of the normal control, the torque gradual-changing section 103 gradually changes the torque level from 100% to 0% (Step S4), thereafter the above operations are repeated until the fade processing ends (Step S5).

As well, the command value gradual-change of the position/speed control and the level gradual-change of the torque control in a fade section (a gradual-change time) may be in any order.

At and after a time point t3 when the fade processing ends, the torque control is switched to the automatic steering (the position/speed control) and then the automatic steering is continued (Step S6).

Thereafter, when the automatic steering command is turned “ON” (a time point t4), or when a driver steers the handle during the automatic steering such that the steering torque Ts exceeds a certain threshold and the automatic steering command is turned “OFF” (the time point t4), the automatic steering is completed (Step S10) and the fade processing is started (Step S11). Also in this case, “OFF” of the automatic steering command is outputted from the automatic steering execution judging section 120. In this way, the steering-angle command value gradual-changing section 100 gradually changes the post-gradual change steering-angle command value θm of the position/speed control from the steering angle command value θt to the actual steering angle θr (Step S12) and the torque gradual-changing section 103 gradually changes the torque level from 0% to 100% (Step S13). This fade processing is continued until a time point t5 (Step S14). At and after the time point t5 when the fade processing ends, the automatic steering is switched to the torque control of the normal steering (Step S15).

Note that, a fading characteristic of the steering angle command value in the position/speed control is represented by an exponential curve while the torque gradual-change in the torque control is represented by a linear line in FIG. 16, however, these may be freely tuned according to handling feeling. Further, a term between the time point t3 and the time point t4 in FIG. 16 is an automatic steering section with a deviation “0”.

Exemplary operations of the automatic steering execution judging section 120 is as shown in the flowchart of FIG. 15. The calculating section 121 in the automatic steering execution judging section 120 is inputted with the steering angle command value θtc from the CAN or the like (Step S20) and calculates the angular speed ωtc and the angular acceleration αtc based on the steering angle command value θtc (Step S21). The angular speed ωtc and the angular acceleration αtc are inputted into the map judging section 122, and the vehicle speed Vs is also inputted into the map judging section 122 (Step S22). The map judging section 122 first judges whether the steering angle command value θtc corresponding to the vehicle speed Vs is within the range of the characteristic values of the judging map #1 shown in FIG. 12A, that is, whether the steering angle command value θtc is below the characteristic line in FIG. 12A (Step S23). If the steering angle command value θtc is within the range of the characteristic values of the judging map #1, next whether the angular speed ωtc corresponding to the vehicle speed Vs is within the range of the characteristic values of the judging map #2 shown in FIG. 12B, that is, whether the angular speed ωtc is below the characteristic line in FIG. 12B is then judged (Step S24). If the angular speed ωtc is within the range of the characteristic values of the judging map #2, whether the angular acceleration αtc corresponding to the vehicle speed Vs is within the range of the characteristic values of the judging map #3 shown in FIG. 12C, that is, whether the angular speed ωtc is below the characteristic line in FIG. 12C is then judged (Step S25). If all of the judging targets are within the range of the respective characteristic values, the automatic steering execution judging section 120 turns “ON” the automatic steering command (Step S31) and outputs the steering angle command value θtc as the steering angle command value θt for inputting to the steering-angle command value gradual-changing section 100 (Step S32).

Further, when the steering angle command value θtc corresponding to the vehicle speed Vs is not within the range of the characteristic values of the judging map #1 shown in FIG. 12A at the above Step S23, or the angular speed ωtc corresponding to the vehicle speed Vs is not within the range of the characteristic values of the judging map #2 shown in FIG. 12B at the above Step S24, or the angular acceleration αtc corresponding to the vehicle speed Vs is not within the range of the characteristic values of the judging map #3 shown in FIG. 12C at the above Step S25, the diagnosing section 123 compares the number of times when the range is exceeded to a predetermined threshold number of times or compares the length of a time period when the range is exceeded to a predetermined threshold period of time (Step S30). Then, when they do not exceed the thresholds, the operation skips to the above Step S31, where the automatic steering command is turned “ON”. When the number of times or the length of the time period exceeds the thresholds, the automatic steering command is turned “OFF” (Step S33), and the steering angle command value θt is blocked and is not outputted (Step S34).

As well, the order of the aforementioned Steps S23 to S25 may be changed as appropriate.

When the automatic steering command is turned “ON” as shown in FIGS. 17A and 17B (a time point t10), the fade processing is started. The post-gradual change steering-angle command value θm is gradually changed from the actual steering angle θr to the steering angle command value θt. The actual steering angle θr is position/speed-controlled in such a manner as to follow the post-gradual change steering-angle command value θm. Consequently, it is possible to automatically and smoothly change the torque command value of the position/speed control, thereby providing the soft handling feeling to the driver.

As well, FIG. 17B shows that a deviation in position is represented as torque.

On the other hand, even when the excessive variations in the steering torque occur after a time point t21 upon the fade processing of the switching from the automatic steering to the torque control (a time point t20) as shown in FIGS. 18A and 18B, the excessive variations in the steering torque is automatically compensated by the position/speed control since the post-gradual change steering-angle command value θm is gradually changed from the steering angle command value θt to the actual steering angle θr. This prevents the driver from losing control of the handle. That is, as shown in FIG. 18A, in the present invention, since the actual steering angle θr is position/speed-controlled in such a manner as to follow the post-gradual change steering-angle command value θm, an occurrence of a peak is delayed and the position/speed control torque command value Tp is generated according to a difference between the post-gradual change steering-angle command value θm and the actual steering angle θr, and then smoothly converges. In the conventional control, however, since the gradual-change starts from a peak of the torque as shown in a broken line in FIG. 18A, the convergence is not smooth. Moreover, a position θr where torque (acceleration) is integrated twice has a trace as in the broken line shown in FIG. 18A and the handle thus moves more.

Furthermore, in another embodiment of the present invention, as compared to the fade processing time (e.g. 1000 [ms]) to perform the fade processing from the torque control of the normal steering to the position/speed control of the automatic steering, the fade processing time (e.g. 100 [ms]) from the position/speed control to the torque control is set shorter as shown in FIG. 19. As a result of this, the control is switched over relatively slowly so that the driver does not feel uncomfortable upon the fade processing from the torque control to the position/speed control, and the control can be switched over in a short period of time and intention of the driver can be promptly conveyed for avoiding danger upon the fade processing from the position/speed control to the torque control.

In the present invention as further shown in FIG. 20, an external disturbance observer 150 to compensate inertia or friction of the handle is provided in the position/speed control section 101 so that a handle manual-input of the driver is not prevented. Further, the external disturbance observer 150 also functions as a torque sensor that detects the handle manual-input at a high speed by estimating a torque input by the driver based on the motor current.

The position/speed control section 101 in FIG. 10 comprises a position/speed feedback controller 170 and the external disturbance observer 150 illustrated in FIG. 20. That is, an input of the position/speed control section 101 is the post-gradual change steering-angle command value θm and an output therefrom is the position/speed control torque command value Tp, and state feedback variables are the steering angle θr and the steering angular speed ωr. The position/speed feedback controller 170 comprises a subtracting section 171 to obtain a steering angle deviation between the post-gradual change steering-angle command value θm and the steering angle θr, a position controller 172 to position-control the steering angle deviation, a subtracting section 173 to obtain a speed deviation between the angular speed from the position controller 172 and the steering angular speed ωr, and a speed controller 174 to speed-control the speed deviation. An output from the speed controller 174 is adding-inputted into a subtracting section 154 in the external disturbance observer 150. Further, the external disturbance observer 150 comprises a steering inverse model 151 of a controlled object that is represented by a transfer function “(J₂·s+B₂)/(τ·s+1)”, a low pass filter (LPF) 152 of a transfer function “1/(τ·s+1)” that is inputted with the position/speed control torque command value Tp and limits a band thereof, a subtracting section 153 to obtain an external-disturbance estimation torque Td*, and a subtracting section 154 to output the position/speed control torque command value Tp by subtraction.

A steering system 160 subjected to the controlled object comprises an adding section 161 to add an unknown external disturbance torque Td to the position/speed control torque command value Tp, a steering system 162 represented by a transfer function “1/(J₁·s+B₁)”, and an integral section 163 to integrate (1/s) the angular speed ωr from the steering system 162 and to output the steering angle θr. The steering angular speed ωr is fed back to the position/speed feedback controller 170 and is also inputted into the integral section 163. The steering angle θr is fed back to the position/speed feedback controller 170.

The symbol “J₁” in the transfer function represents the inertia in the steering system 162, “B₁” represents the friction in the steering system 162, “J₂” represents the inertia in the steering inverse model 151, “B₂” represents the friction in the steering inverse model 151, and “τ” represents a predetermined time constant. These have relationships represented by the following equations 1 and 2. J ₁ ≥J ₂  (Equation 1) B ₁ ≥B ₂  (Equation 2)

The external disturbance observer 150 estimates the unknown external disturbance torque Td base on a difference between outputs of the steering inverse model 151 and the LPF 152 and obtains the external-disturbance estimation torque Td* as an estimation value. The external-disturbance estimation torque Td* is subtracting-inputted into the subtracting section 154, and it is possible to realize a robust position/speed control by subtracting the external-disturbance estimation torque Td* from an output of the speed controller 174. However, the robust position/speed control results in contradiction that the handle cannot be stopped even with intervention by the driver. In order to improve this point, the inertia J₂ and the friction B₂ smaller than or equal to the inertia J₁ and the friction B₁, respectively, which the steering system 162 actually has, are inputted as the steering inverse model 151. As a result of this, the inertia and the friction of the handle that the driver feels becomes seemingly smaller. This allows the driver to easily intervene in the automatic steering by steering.

Moreover, by monitoring the external-disturbance estimation torque Td* in the external disturbance observer 150, it is possible to detect the steering torque of the driver instead of the torque sensor. Especially, when the torque sensor uses digital signals, detection of steering intervention by the driver may be delayed due to influence of communication delay or other reasons. Similarly to the torque sensor, when the external-disturbance estimation torque Td* exceeds a threshold value for a predetermined period of time, the steering intervention may be determined to be performed and the fade processing may be performed.

FIGS. 21A and 21B are diagrams illustrating characteristics of the angle and the torque, respectively, in the fade processing from the position/speed control to the torque control when the external disturbance observer 150 is provided. The driver turns the handle in an opposite direction to a direction of the steering angle command value θt in the automatic operation and releases the handle when the automatic steering is turned “OFF” (the fade processing is started). In FIGS. 21A and 21B, the characteristics of the external disturbance observer 150 in a case where the inertia and the friction satisfy “J₁>J₂” and “B₁>B₂” and a case where “J₁=J₂” and “B₁=B₂” are satisfied are illustrated. FIG. 21A is a diagram illustrating exemplary variations in the actual steering angle θr when the external disturbance observer 150 is provided. FIG. 21B is a diagram illustrating exemplary variations in the steering torque Ts and the position/speed control torque command value Tp when the external disturbance observer 150 is provided.

Providing the external disturbance observer 150 allows for providing a smoother operation feeling, thereby enabling switching control at a high speed. Smaller inertia and friction facilitate the steering intervention.

EXPLANATION OF REFERENCE NUMERALS

-   1 handle (steering wheel) -   2 column shaft (steering shaft, handle shaft) -   10 torque sensor -   12 vehicle speed sensor -   20, 131 motor -   30 control unit (ECU) -   40 CAN -   41 Non-CAN -   50 automatic steering command unit -   51, 101 position/speed control section -   52, 120 automatic steering execution judging section -   53 torque control section -   54 torque command value gradual-change switching section -   100 steering-angel command value gradual-changing section -   102 torque control section -   103 torque gradual-changing section -   121 calculating section -   122 map judging section -   123 diagnosing section -   130 current control system -   150 external disturbance observer 

The invention claimed is:
 1. An electric power steering apparatus including a torque sensor to detect a steering torque and a motor control unit to control a motor that applies an assist torque to a steering system of a vehicle, comprising: a function to switch a control system of said motor between a torque control system to control a motor output torque and a position or speed control system to control a steering angle of a steering in accordance with a predetermined switching trigger; and an external disturbance observer to compensate inertia and friction of a handle, wherein, when an automatic steering command is turned ON, a fade processing is started and a post-gradual change steering-angle command value in a position or speed control is gradually changed from an actual steering angle to a steering angle command value, and wherein said post-gradual change steering-angle command value in said position or speed control is gradually changed by an exponential curve, and a level of the assist torque is gradually changed linearly, and wherein, a fade processing time 1 from said torque control system to said position or speed control system is different from a fade processing time 2 from said position or speed control system to said torque control system.
 2. The electric power steering apparatus according to claim 1, wherein said predetermined switching trigger is ON or OFF of the automatic steering command.
 3. The electric power steering apparatus according to claim 1, wherein said predetermined switching trigger is ON or OFF of a switching command given by an internal judgment of said steering torque.
 4. The electric power steering apparatus according to claim 1, wherein said predetermined switching trigger is performed by an automatic steering execution judging section.
 5. The electric power steering apparatus according to claim 1, wherein a fade characteristic of said fade processing can be freely tuned.
 6. The electric power steering apparatus according to claim 1, wherein said fade processing time 2 is shorter than said fade processing time
 1. 7. The electric power steering apparatus according to claim 4, wherein said automatic steering execution judging section comprises: a calculating section to calculate an angular speed and an angular acceleration by inputting a steering angle command value; a map judging section to judge each of said steering angle command value, said angular speed and said angular acceleration with a judging map corresponding to a vehicle speed; and a diagnosing section to diagnose based on a judgement result from said map judging section.
 8. The electric power steering apparatus according to claim 1, wherein said external disturbance observer estimates an external-disturbance estimation torque from a difference between an output of a steering inverse model of the steering system and an output of a low pass filter (LPF) to limit a band.
 9. The electric power steering apparatus according to claim 8, wherein values of inertia and friction of said steering system are greater than or equal to values of inertia and friction of said steering inverse model, respectively.
 10. An electric power steering apparatus including a torque sensor to detect a steering torque and a motor control unit to control a motor that applies an assist torque to a steering system of a vehicle, comprising: a function to switch a control system of said motor between a torque control system to control a motor output torque and a position or speed control system to control a steering angle of a steering in accordance with a predetermined switching trigger; and an external disturbance observer to compensate inertia and friction of a handle, wherein, when an automatic steering command is turned OFF, a fade processing is started and a post-gradual change steering-angle command value in a position or speed control is gradually changed from a steering angle command value to an actual steering angle, and wherein said post-gradual change steering-angle command value in said position or speed control is gradually changed by an exponential curve, and a level of the assist torque is gradually changed linearly, and wherein a fade processing time 1 from said torque control system to said position or speed control system is different from a fade processing time 2 from said position or speed control system to said torque control system.
 11. The electric power steering apparatus according to claim 10, wherein said predetermined switching trigger is ON or OFF of the automatic steering command.
 12. The electric power steering apparatus according to claim 10, wherein said predetermined switching trigger is ON or OFF of a switching command given by an internal judgment of said steering torque.
 13. The electric power steering apparatus according to claim 10, wherein said predetermined switching trigger is performed by an automatic steering execution judging section.
 14. The electric power steering apparatus according to claim 10, wherein a fade characteristic of said fade processing can be freely tuned.
 15. The electric power steering apparatus according to claim 10, wherein said fade processing time 2 is shorter than said fade processing time
 1. 16. The electric power steering apparatus according to claim 13, wherein said automatic steering execution judging section comprises: a calculating section to calculate an angular speed and an angular acceleration by inputting a steering angle command value; a map judging section to judge each of said steering angle command value, said angular speed and said angular acceleration with a judging map corresponding to a vehicle speed; and a diagnosing section to diagnose based on a judgement result from said map judging section.
 17. The electric power steering apparatus according to claim 10, wherein said external disturbance observer estimates an external-disturbance estimation torque from a difference between an output of a steering inverse model of the steering system and an output of a low pass filter (LPF) to limit a band.
 18. The electric power steering apparatus according to claim 17, wherein values of inertia and friction of said steering system are greater than or equal to values of inertia and friction of said steering inverse model, respectively. 